The present invention relates to a method for operating a motor vehicle having an automatic transmission during and/or in the time of a selection action for a driving-mode range.
To activate an automatic transmission, it is conventional, for example, as described in German Published Patent Application No. 198 07 357, to transfer a driver""s requirement for a change in the driving-mode range, for example, into one of the driving-mode ranges P (parking mode), R (reverse travel), N (neutral position) or D (forward travel), to a control unit via a selection device designed as a selector lever or driving-mode selector lever. A suitable, in particular electrical or mechanical, change signal is generated in the selection device or the control unit. According to the change signal, a change in the driving-mode range of the automatic transmission is achieved, for example, by a change in the operating state of a clutch or brake of the automatic transmission or the release of a parking brake. This change in the driving-mode range leads to a change in the force flux and/or in the transmission ratio between the crankshaft of the drive assembly and the vehicle wheels. The change results in a change in the drive torque acting on the vehicle wheels, in the most unfavorable case, in a jump in the drive torque. The drive train, e.g., releasably, connecting the drive assembly to the vehicle wheels forms a continuum torsional oscillator or, in a simplified illustration, a torsional oscillator chain having multiple degrees of freedom.
Oscillations of the drive train occur because of the change in the drive torque. This results in accelerations of the motor vehicle fluctuating in time, so-called jolting oscillations, which are detrimental to, e.g., start-up, comfort. Further adverse effects may be impairments of driving safety, increased pitching movements or loss of track stability in the start-up range, increased slip, increased component wear and greater component dimensioning or, in the worst case, component failure. Other consequences of an undesirable oscillatory behavior are noise and vibration problems.
German Published Patent Application No. 34 04 154 describes a regulating device, in which, in order to damp oscillations, a controlling variable is determined from operating parameters in a correcting element. This controlling variable is supplied to a mixture-forming system of the internal combustion engine driving the motor vehicle. The mixture-forming system is operated, as a function of this controlling variable, so that the drive torque of the internal combustion engine is changed so that oscillations in the drive train are damped or suppressed. In a spark-ignition internal combustion engine, this is performed by the controlling variable influencing the throttle valve of the internal combustion engine.
German Published Patent Application No. 40 09 791 describes a method for damping oscillations of the type described above, in which a controlling variable determined in a correcting device and changing the drive torque of the internal combustion engine is supplied to the internal combustion engine as soon as oscillations occur. So that oscillations of this kind can be detected reliably and also damped quickly and effectively, rotational-speed gradients are determined from rotational-speed information and used for oscillation assessment. Moreover, by an in-phase change-over from the ignition times of a first ignition characteristic diagram stored in an ignition-time control apparatus to the corresponding ignition times of a second ignition characteristic diagram stored in the ignition-time control apparatus, the drive torque of the internal combustion engine is changed so that the oscillations in the drive train of the internal combustion engine are damped.
German Published Patent Application No. 197 33 472 describes a method for damping oscillations during the positive and negative acceleration of motor vehicles having manual-shift transmission and having an accelerator pedal acting on an engine and intended for stipulating the engine torque. In this method, when the accelerator pedal stipulates changes of the engine torque in jumps from an initial torque to a target torque, this engine torque is changed in two steps. In the first step, an intermediate engine torque is stipulated in a jump and, starting from an initial value of the vehicle acceleration, leads to a vehicle acceleration of which the first oscillation which is established has a peak value which corresponds to the new stationary value of the vehicle acceleration corresponding to the target torque. When this peak value is reached, in the second step, the target torque is stipulated in a jump. Oscillations can thereby be prevented effectively in all the operating states, without action being taken on the ignition and without the response behavior of the vehicle being appreciably impaired.
It is an object of the present invention to provide a method for the avoidance or reduction of drive-train oscillations caused as a result of a selection action for a driving-mode range.
The above and other beneficial objects of the present invention are achieved by providing a method as described herein.
During or in the time of the selection action for the driving-mode range, at least one braking device is actuated in an automated manner by the or a control unit.
The braking device may include at least one braking device of any desired configuration that is arranged at any desired location on the drive train. For example, the braking device may include a wheel brake, a differential brake, a transmission brake or a generator driven by part of the drive train. Alternatively or additionally, the braking device may include a brake or clutch of the automatic transmission, by which brake or clutch it becomes possible for transmission-side part-regions of the drive train to be braced against one another or relative to the housing.
The effective duration of automated braking actuation may be, for example, in the range of milliseconds or seconds, for example 300 to 600 milliseconds, 600 to 1000 milliseconds or one or more seconds.
The present invention makes use of the fact that, with the automated introduction of friction in the braking device, additional damping in an amount capable of being influenced is introduced into the oscillatory system, with the result that oscillations of the drive train which occur due to the change in the drive torque may be damped effectively. Furthermore, in the case of a change in the braking torque applied in automated manner, a counteroscillation may be excited which is superposed in the drive train on the oscillation caused by the change in the drive torque. If there is a favorable configuration of the phase relationship, oscillation amplitude and oscillation frequency of the counteroscillation, the superposition of the forms of oscillation results in the resulting form of movement being reduced and, in the most favorable case, eliminated.
Due to the configuration according to the present invention, engine-side action measures, such as action on the ignition or action on the throttle valve, during the start-up process may be eliminated, reduced or supplemented. The braking action according to the present invention may occur more quickly than other conventional action measures if the control unit and the braking device are suitably configured.
In the simplest case, a movement of the motor vehicle is avoided despite, for example, abrupt or rapid, selection movement, for example, from N to D or R, by the automated generation of a fixed braking torque, the fixed braking torque subsequently being released in an automated manner, so that the drive torque is, e.g., gradually, issued to the vehicle wheels, with oscillations of the vehicle acceleration being eliminated.
Alternatively, the braking torque applied in an automated manner during or in the time of the selection action may be lower than the drive torque transmitted to the vehicle wheels by the drive assembly, so that a reduced residual drive torque remains for the acceleration of the motor vehicle. The result of this is that, for example, in the case of an initially sharp rise of the drive torque with a subsequent more gradual rise of the latter, if the braking torque is suitably selected the residual drive torque is transferred to the vehicle wheels for acceleration only when the braking torque is reached, in the most favorable case only within the range of the more gradual rise of the drive torque. In this case, the braking torque forms a threshold value below which torque jumps may not lead to the start-up comfort being impaired.
The braking torque generated by the braking device constitutes an effective damping of the forms of oscillation of the drive train or of the start-up acceleration because of the frictional forces which are always counter to the direction of movement of the drive train and are substantially independent of the speed of the drive train.
The method according to the present invention may be implemented in a simple manner, for example, with components which are present in any case. By the control unit, in particular, brake and/or traction systems, etc., present in any case, may be activated and signals from these utilized. The control unit, a transmission control device and control units from the brake or traction systems may be configured separately from one another and communicate with one another, in particular via a CAN bus, or be configured as a multi-functional unit.
According to a further aspect of the present invention, a braking torque generated according to a brake pedal or a parking brake, for example, in the parking position, during or in the time of the selection action is at least partially maintained in an automated manner in spite of the release of the brake pedal or the parking brake. This constitutes a particularly efficient example embodiment of the method according to the present invention, since, for example, a brake pressure present in any case in the brake cylinders may be utilized and therefore there is no need for a time-consuming and/or high-energy hydraulic pressure build-up in the brake cylinders.
A braking torque generated in an automated manner in the braking device may have a curved profile. This profile may be made smooth at the commencement of automated braking actuation and at the end of this or have a kink or a jump. The profile and the transitions at the commencement and the end are matched to the changes in the drive torque and to the dynamics of the torsional oscillator chain so that the oscillation which is established in the vehicle acceleration is minimal. In this case, any desired curve profiles, such as, for example, rectilinear, ramp-like, polynomial or harmonic profiles, may be considered.
In the method according to the present invention, the braking torque generated in an automated manner may be applied according to a form of movement of the drive train. For this purpose, for example, the form of movement of the drive train or the vehicle acceleration is detected by measurement. The drive torque causing the oscillation excitation may be detected. In the case of a known transmission behavior or known transmission function, the oscillation resulting without a braking action may be determined and the braking-torque profile necessary for a reduction in oscillation be ascertained a priori or during operation. For a change in the braking torque, the alternating component of a signal may be processed and an increased braking torque may be generated in the phases of an excessive acceleration of the motor vehicle. By contrast, in the phases of desired or medium acceleration the braking action may be reduced, so that the stipulation of the driving-mode range, for example, a desired start-up, may be followed on average undamped and without delay, along with a simultaneous damping of the acceleration fluctuations in time.
According to a further aspect of the present invention, the braking torque generated in an automated manner may be applied according to a characteristic diagram. This characteristic diagram takes into account, for example, summer and winter operating ranges of the automatic transmission, drive conditions, changed due to temperature, in the transmission or engine or between the road and vehicle wheels and also other operating parameters, such as rotational speeds, component age, fuel quality, vehicle mass, including or excluding load, and/or ambient parameters such as the road quality or road gradient. Output variables of the characteristic diagram may be, for example, braking-torque profiles, the size of a jump or kink, the duration of the braking torque, curve-profile parameters or oscillation durations and oscillation amplitudes for braking-torque profiles fluctuating in time. The method according to the present invention may thus refer to empirically determined experimental values or to physical dependencies determined. A procedure of this kind has proved to be particularly reliable in practice.
The features mentioned above and those yet to be explained below may be used not only in the combination specified in each case, but also in other combinations or alone, without departing from the scope of the present invention.